1. Field of the Invention
The present invention relates to a thin film bulk acoustic resonator (also referred to as “FBAR” hereinafter) for use in a filter and duplexer for communication in the RF (Radio Frequency) band. More particularly, the present invention relates to an air-gap type FBAR and method for fabricating the same, and a filter and a duplexer using the air-gap type FBAR, whose fabricating process is simplified and made more stable by using a LCP (Liquid Crystal Polymer) thin film.
2. Description of the Related Art
Recently, wireless mobile telecommunication technology has been rapidly advancing. Such mobile telecommunication technology requires a variety of RF parts capable of efficiently delivering information in a limited frequency band. Particularly, a filter among RF parts is essential for use in mobile telecommunication technology, which makes high-quality communication possible, by selecting a signal needed by a user among numerous broadcasting waves or by filtering a signal that is to be transmitted.
Currently, the RF filter most widely used for wireless communication is a dielectric filter and a surface acoustic wave (SAW) filter. The dielectric filter is advantageous in that it has high permittivity, low insertion loss, high temperature stability, and strongly resists inner vibration and inner impulse. The dielectric filer, however, has limitations in small-sizing and application to MMIC (Monolithic Microwave Integrated Circuit) which are recent trends in the technology development. On the other hand, the SAW filter is advantageous in that it is small-sized compared to the dielectric filter and signal processing is simple, the circuit is simple, and mass production is possible by using a semiconductor process. Also, the SAW filter is well adapted for outputting and inputting high-quality information, because rejection within the pass band is high as compared to the dielectric filter. The SAW filter, however, has a weak point in that IDT (Inter-Digital Transducer) line width is limited to 0.5 μm, because the process for exposing with an ultraviolet ray is included in the process for fabricating the SAW filter. Consequently, it is difficult to cover the extremely high frequency band (higher than 5 GHz) using the SAW filter and it is also difficult to construct a SAW filter together with a MMIC structure on a single chip of a semiconductor substrate.
To overcome such limitations and problems, FBAR capable of being completely integrated with MMIC together with other active elements on existing semiconductor substrates (e.g., Si, GaAs), has been proposed.
Since FBAR, which is a thin film element, is of low cost and small-sized and allows for a high quality coefficient, FBAR can be used for wireless communication in a variety of frequency bands (900 MHz˜10 GHz) and radar for military use. Also, FBAR has properties such that small sizing is possible in a size of one to several hundredths that of a dielectric filter, and insertion loss is very small as compared to the SAW filter. Therefore, FBAR could be applied to a MMIC requiring high stability and a high quality coefficient.
FBAR having a stacked structure comprising an upper electrode/piezoelectric material/lower electrode is fabricated by a semiconductor process. A piezoelectric phenomenon is generated so that resonance may occur in a predetermined frequency band, and a volume wave is utilized. At the same moment, if the frequency of the volume wave becomes identical with the frequency of an input electric signal, a resonance phenomenon occurs. A resonator using such resonance phenomenon is realized in a FBAR filter through development of electric coupling, and further a duplexer using FBAR may also be realized.
In the meantime, FBAR structure has been studied in variety of ways to date. In case of a membrane type FBAR, silicon oxide film (SiO2) is deposited on a substrate, and a membrane layer is formed on the opposite side of the substrate through a cavity part formed by isotropic etching. Next, a lower electrode is formed on the upper part of the silicon oxidation film, a piezoelectric material is deposited on the upper part of lower electrode by a RF magnetron sputtering method to form a piezoelectric layer, and an upper electrode is formed on the upper part of the piezoelectric layer.
The above described membrane type FBAR is advantageous in that dielectric loss of the substrate is low due to the cavity, and power loss is small. The membrane type FBAR, however, is problematic in that the size occupied by the element is large due to orientation of the silicon substrate. Also, yield decreases due to breakage upon subsequent packaging because structural stability is low. Therefore, to reduce loss by the membrane and to simplify the process for fabricating the element, an air-gap type and a bragg-reflector type FBAR have recently been proposed.
The bragg-reflector type FBAR is fabricated such that a material, whose elastic impedance difference is large, is deposited alternately on the substrate so that a reflecting layer is formed, and a lower electrode, a piezoelectric layer, and an upper electrode are sequentially stacked. According to the bragg-reflector type FBAR fabricated in this manner, elastic energy that has passed through the piezoelectric layer cannot be delivered in the substrate direction but is completely reflected at the reflecting layer, whereby efficient resonance is potentially generated. Such bragg-reflector type FBAR is advantageous in that it has a solid structure and there is no stress due to bending, but is disadvantageous in that it is difficult to form the reflecting layer of more than four layers whose thickness is accurate for total reflection, and much time and cost are required for its fabrication.
On the other hand, a conventional air-gap type FBAR having a structure in which the substrate is separated from the resonance part using an air gap instead of a reflecting layer, realizes FBAR by forming a sacrificial layer 110 by isotropic etching of the surface of the silicon substrate 100, surface polishing using CMP (Chemical Mechanical Planarization), then sequentially depositing an insulating film 120, a stacked structure 130 including a lower electrode 133, a piezoelectric layer 135, and an upper electrode 137, removing the sacrificial layer 110 through a via hole, and forming an air gap 140 as shown in FIG. 1. A second substrate 150 having a cavity part 160 in its lower surface is then joined to the silicon substrate 100.
However, the method for fabricating FBAR according to the conventional art requires a CMP process, thereby increasing complexity in terms of the process and increasing cost. Also, wet etching is used for forming the air gap in the method of the conventional art, and in that case, it is difficult to remove the etching solution. If the etching solution is not completely removed, the element is deteriorated due to constant action of the etching solution, thereby causing a change in resonance frequency. On the other hand, in the case of dry etching, physical impulse is exerted on the element by ions and molecules in a plasma state using existing plasma dry etching techniques, and the element is deteriorated due to high temperature.